In order to allow a semiconductor device to be high in breakdown voltage and low in loss and to be used in a high-temperature environment, silicon carbide has recently increasingly been adopted as a material forming a semiconductor device. Silicon carbide is a wide band gap semiconductor greater in band gap than silicon which has conventionally widely been used as a material forming a semiconductor device. Therefore, by adopting silicon carbide as a material forming a semiconductor device, a higher breakdown voltage and a lower on-resistance of a semiconductor device can be achieved. A semiconductor device in which silicon carbide has been adopted as a material is also advantageous in that lowering in characteristics during use in a high-temperature environment is less than in a semiconductor device in which silicon has been adopted as a material.
For example, in a metal oxide semiconductor field effect transistor (MOSFET) and an insulated gate bipolar transistor (IGBT) among semiconductor devices in which silicon carbide is employed as a material, whether a current which is to flow between two electrodes is allowed or not allowed to flow can be controlled by controlling whether or not an inversion layer is formed in a channel region with a prescribed threshold voltage being defined as a boundary.
For example, Mitsuo Okamoto et al., “Reduction of Instability in Vth of 4H-SiC Carbon Face MOSFETs,” the 59th Spring Meeting, Proceedings, The Japan Society of Applied Physics, Spring 2012, pp. 15-309 (NPD 1) has pointed out that a silicon carbide MOSFET suffers from fluctuation in threshold voltage due to gate bias stress. This document discloses a method of annealing a silicon carbide substrate having a gate oxide film formed in a hydrogen atmosphere in order to lessen fluctuation in threshold voltage.